In many applications in which coordinated control for driving one or more motors on multiple axes is required, such as in robotics applications, machine tools, and the like, multi-turn absolute position information (i.e., information from which motor position from a known reference point, normally established at the time of machine setup, can be determined) is often essential. If the motor position relative to the machine home or other reference is unknown, the controller must drive the motor back to its reference point each time power is applied to establish the initial position before normal operation can commence. Homing to synchronize a multi-axis system under such circumstances is often highly inconvenient and time consuming, especially when constraints are present in the travel path.
Two basic types of signal feedback position transducers are currently in general used in motor drive systems. Incremental position transducers, such as optical encoders, typically provide two channel outputs in quadrature configuration; changes in position are determined incrementally by decoding the quadrature state transitions that occur in the channel signals. Absolute position transducers provide signals from which the specific position of a motor shaft can be determined at any point within a single motor revolution.
Neither of the conventional transducers described provides multiple-revolution absolute position information. As presently configured, therefore, they are not adequate for most current applications, which require numerous motor revolutions to achieve a desired range of travel. Consequently, electronic counters are commonly used to record absolute motor position during multiple revolutions.
In order to maintain absolute multi-turn position information when the main power for driving the motor has been interrupted, it is of course necessary to supply power to the circuit from a secondary source; DC battery backup circuits are usually used to accommodate AC power failures. In normal operation, however, the power consumption of most position transducers is very high, typically on the order of 50-200 milliamperes at 5V or 12V depending upon the particular kind of transducer involved. The energy capacity of the battery employed therefore represents a fundamental limitation upon the period during which an AC power failure, or machine downtime, can be accommodated without losing the ability to track the position of a motor. Consequently, power conservation is a matter of primary concern in such systems.
The use of inductive sensors in motor control systems is well known and is described, for example, in Horber U.S. Pat. No. 4,687,961 and Horber and Vu U.S. Pat. No. 5,329,195 (the specifications of which patents are hereby incorporated hereinto by reference). Such sensors afford, among other benefits, high levels of accuracy, reliability, and robustness.
Hibino et al. U.S. Pat. No. 5,187,724 provides an absolute position detecting device which employs a battery-powered auxiliary power supply. An electronic multi-shaft absolute position detecting device is disclosed in Tsai et al. U.S. Pat. No. 5,287,285, in which a spare power circuit enables continuous detection of position signals, despite power supply disruptions. Kyoizumi U.S. Pat. No. 5,412,317 provides a position detector in which an absolute position sensor (preferably of the magnetostrictive line type) and an incremental position sensor (preferably of the magnetic induction type) are utilized in combination.
It is a broad object of the present invention to provide a novel control system, and a novel control method, for the continuous, non-volatile tracking of the position of a motor armature.
More specific objects of the invention are to provide such a system and method wherein a secondary battery-powered backup circuit is provided to maintain position-tracking capability, despite loss or termination of power in the primary power supply, and wherein power consumption is conserved so as to extend the potential duration of the backup mode.
Another object of the invention is to provide a dual-sensor position transducer suitable for use in such a tracking system and method, where the transducer is of compact, and relatively incomplex and inexpensive, construction.
It has now been found that certain of the foregoing and related objects of the invention are attained by the provision of a control system for the continuous, non-volatile tracking of the position of a motor armature, which system comprises motor armature position data acquisition and storage means, a primary power supply circuit for delivering drive current to the motor, a battery-powered secondary circuit, and means for operatively connecting the primary power supply circuit to the data acquisition and storage means. The data acquisition and storage means includes an armature position transducer, at least one electronic data processing unit, and sampling means for causing the electronic data processing unit to intermittently access the position transducer, for defined time periods and at controlled variable frequency of access. The means for operatively connecting serves to connect automatically the secondary circuit to the data acquisition and storage means, so as to enter and implement the backup mode, such connection occurring only upon disruption of power in the primary power supply circuit. The frequency of access caused by the sampling means in the backup mode varies in a direct relationship to the speed of the motor armature, as detected by the position transducer.
In most instances the active periods, during which the position transducer draws full operating power, are limited substantially to the defined periods of access (albeit generally of slightly longer duration). All of the active periods will usually be of the same length, and the frequency of access will advantageously be varied by adjusting the duration of inactive periods which intervene between the active periods and are all also of equal duration. The position transducer draws substantially less than full operating power during the inactive periods, such that power is applied to the position transducer in accordance with a variable duty cycle. The primary power supply circuit will normally be disconnected from the data acquisition and storage means when the secondary circuit is connected thereto, and the position transducer will usually draw only nominal power during the inactive periods. Preferably, the data acquisition and storage means will comprise primary and secondary electronic data processing units, operatively inconnected and having relatively high computing capability and relatively low computing capability respectively.
The positon transducer will, in most instances, comprise at least one sensor selected from the group consisting of encoders and inductive sensors, constructed for the detection of rotary armature movement. The sensor (or each of a plurality of sensors) will preferably generate at least one set of at least two electrically out-of-phase signals that vary sinusoidally so as to represent trigonometric functions from which the angle of a shaft of a motor can be determined. The signal set will normally consist of two signals that bear a 90xc2x0 phase relationship to one another, with the trigonometric functions being sine and cosine functions; the sampling means will advantageously comprise a zero crossing point detector and, in particular, a quadrature detector.
In especially preferred embodiments the position transducer will comprise a second sensor, with the xe2x80x9conexe2x80x9d signal set being a first signal set representing a minimal whole number of electrical cycles of the xe2x80x9cat least two signals,xe2x80x9d and with the second sensor being configured to generate a second signal set representing a multiple whole number of the minimal number of the electrical cycles; in such embodiments only the xe2x80x9cfirstxe2x80x9d signal will normally be utilized for causing the frequency of access to vary in the backup mode.
In particularly desirable embodiments of the system, components of plural sensors will be integrated into a substantially circular rotor body. Such a rotor body may be of generally wedge-shaped cross section, so as to induce (by virtue of the variation of ferromagnetic material presented to the several magnetics fields) a single electrical cycle per revolution and to provide the xe2x80x9cfirstxe2x80x9d signal, and will have peripheral structure configured for inducing multiple electrical cycles per revolution, thereby providing the xe2x80x9csecondxe2x80x9d signal. Alternatively, the rotor body may be mounted eccentrically of the stator, such that the varying gap therebetween (and the consequential variation in magnetic permeability presented) will again cause a single electrical cycle to be induced per revolution; it will be appreciated that other structures may generate two or more electrical cycles to constitute the xe2x80x9cfirstxe2x80x9d signal.
Certain objects of the invention are attained by the provision of a method for the continuous, non-volatile tracking of the position of a motor armature, utilizing a control system of the character herein described. In carrying out the method the primary power supply circuit, used for delivering drive current to the motor, is normally operatively connected to the data acquisition and storage means. Upon disruption of power in the primary power supply circuit, the secondary, backup circuit is connected automatically to the data acquisition and a storage means, and the frequency of position data sampling is caused to vary in a direct relationship to the speed of the motor armature.
Various features of the system described are utilized to implement specific aspects of the method of the invention. In particularly preferred embodiments, the position transducer will comprise at least two sensors, one being configured to generate a first signal set, representing a minimum number electrical cycles per revolution (advantageously, only a single cycle), and the other being configured to generate a second signal set representing ten or more electrical cycles per revolution, with only the first signal set being utilized for causing variation in the frequency of access to position data.
Additional objects of the invention are attained by the provision of a control system for tracking the position of a motor armature, which comprises armature position data acquisition and storage means, an armature position transducer, at least one electronic data processing unit, and sampling means for causing the electronic data processing unit to intermittently access the position transducer, for defined time periods and at controlled frequency of access. The position transducer utilized comprises first and second inductive sensors, one of which is configured to generate a first signal set and the other of which is configured to generate a second signal set, each signal set consisting of two sinusoidal waves that are 90xc2x0 out of phase and that represent sine and cosine functions, from which the angle of the shaft of a motor can be determined. The first signal set represents a minimal whole number of electrical cycles of the two signals, and the second signal set represents a multiple whole number of the minimal number of the electrical cycles, the first and second sensors being selectively accessible by the electronic data processing means by way of the sampling means.
Preferably, the number of cycles comprising the second signal set will be at least one order of magnitude larger than the minimal number. The first and second sensors utilized in the system will most desirably comprise a substantially circular rotor body into which components of each sensor are integrated, as hereinbefore and hereinafter described.
Thus, in accordance with the present invention a variable frequency sampling method is applied to efficiently utilize the battery charge in a backup circuit for a motor position transducer. This is done by varying the duty cycle, for accessing the transducer, as a function of the velocity at which the motor armature moves due to the application of external forces. The control cycle optimally consists of two time segments, defined herein as ON time and OFF time, with the ratio of ON time to the total of ON time plus OFF time constituting the duty cycle; needless to say, the smaller the duty cycle value the lower the power consumption level will be. Albeit other schemes may be employed, the duty cycle will most suitably utilize a fixed ON time and a variable OFF time, with a suitable OFF time duration being determined and applied after each ON time interval in which position data are sampled and velocity is sensed and calculated.